Cardiff's automatic promotion hopes are ended as Middlesbrough beat them 3-0 before Norwich won at Portsmouth to move into the Premier League.
Boro were three ahead inside 22 minutes after Leroy Lita's header from Tony McMahon's cross beat Tom Heaton.
Barry Robson's flick from Lita's cross doubled the lead as Boro once again exposed Cardiff's defensive frailties.
Richard Smallwood's close-range third meant Norwich would clinch promotion if they beat Portsmouth.
And indeed Simeon Jackson's winner at Fratton Park then secured a 1-0 Norwich win over Portsmouth to secure a top-flight return after a six-year absence.
So Cardiff's first defeat in nine games and Norwich's subsequent triumph guarantees Norwich a second automatic promotion spot alongside Queens Park Rangers. However, QPR's position depends on the outcome of their FA hearing over breaching league rules.
It was the Bluebirds' heaviest home defeat to Middlesbrough for two years and means they are in the play-offs for a second successive season after 2010's failure in the final.
Cardiff manager Dave Jones had deflected any pre-match suggestions of pressure even though the Welsh club knew nothing less than victory over Middlesbrough would help secure the Championship's second automatic promotion spot.
The tension at the Cardiff City Stadium was exacerbated when the team, who had lost once at home since November, were three goals down inside 22 minutes.
The rout began in the opening 150 seconds when McMahon's deep, hanging right-wing cross was not dealt with by Cardiff's defence and Lita rose above Kevin McNaughton and Dekel Keinan to guide a header home.
What Lita's effort lacked in power, it made up for in direction as it beat goalkeeper Tom Heaton from 12 yards for his 11th of the season, and 99th of his club career.
Lita soon turned provider when he was allowed time by Keinan to fire a low shot across the six-yard box that Boro midfielder Barry Robson poked into the far corner of the net past Heaton.
The Scotsman's fourth goal of the season exposed more abysmal Bluebirds defending as Robson's fellow countryman and newly-crowned Cardiff fans Player of the Year McNaughton was beaten to the ball again.
Captain Craig Bellamy, so often Cardiff's talisman this season, claimed for a penalty when his 25-yard free-kick struck Robson's arm in the wall just inside the area - but referee Gerald Scott ignored the protests.
A poor cross from Jay Bothroyd when Cardiff had men in the danger area, two squandered Seyi Olofinjana openings and a Bothroyd curling shot wide were the best attempts the hosts could offer.
And the Bluebirds' automatic promotion bid looked over when they conceded a third for the first time in a regular-season league game at the Cardiff City Stadium.
Jlloyd Samuel let Marvin Emnes reach the byline, and the former Swansea striker's cutback was smashed into the corner of Heaton's net from 12 yards by Smallwood.
The Boro academy product's first professional goal marked the third successive away game in which Tony Mowbray's side had scored three. It prompted some fuming Cardiff fans to head for the exit.
A Keinan snap-shot from Chris Burke's corner missed the target from 12 yards just after Jones had hauled Samuel off the pitch. Jason Koumas came on, no doubt in the hope of creating a footballing miracle.
Both Bellamy and Olofinjana had goals disallowed for offside either side of an interval that was welcomed by the chorus of boos from home supporters.
Cardiff could not fail to improve in the second half which started by McNaughton almost squeezing Bellamy's inswinging right-wing cross home at the back post - but the defender could not guide his flick goalwards.
The languid Bothroyd fired over before Burke did likewise as Cardiff slumped to their first home defeat to Boro since 1970 - and just their fourth home defeat this season.
Another penalty appeal was turned down late on as Olofinjana's shot was blocked by the arm of Stephen McManus but Boro comfortably held on.
Manager Mowbray insisted pre-match he had wanted his men to be Cardiff's promotion party poopers and it was mission accomplished as Cardiff will need to go through the play-offs if their 49-year top-flight absence is to end this season.